 The primary objective of the cluster mission is to accurately measure the magnetic field along the orbits of the four spacecraft. The magnetic field plays an active role in all physical processes that define the structure and dynamics of magnetospheric phenomena on all scales. With the four-point measurements on cluster, it has become possible to study three-dimensional aspects of space plasma phenomena on scales comparable with the size of the spacecraft constellation and to distinguish temporal and spatial dependencies of small-scale processes. The instrumentation used to measure the magnetic field on the four cluster spacecraft includes magnetometers. The performance of the operational modes used in flight has been presented, as well as the results of preliminary in-orbit calibration of the magnetometers, which show that all components of the magnetic field are measured with an accuracy approaching 0.1 NT. Further data analysis is expected to bring an even more accurate determination of the calibration parameters. Several examples of the capabilities of the investigation have been presented from the commissioning phase of the mission and from the different regions visited by the spacecraft to date, including the tail current sheet, the dusk side magnetopause and magnetosheath, the bow shock and the cusp. The data processing flow and the implementation of data distribution to other cluster investigations and to the scientific community in general have also been described. Key words, interplanetary physics, instruments and techniques, magnetospheric physics, magnetospheric configuration and dynamics, space plasma physics, shock waves. This article was authored by Aballo, C.M. Carr, M.H. Akunya and others. We are article.tv, links in the description below.